1. Technical Field
This invention relates generally to loudspeaker accessories for portable electronic devices, and more particularly to speakerphone accessories for cellular telephones having improved low frequency response.
2. Background Art
One of the biggest problems that loudspeaker designers face is generating satisfactory low end, or bass, response. Specifically, the challenge is generating louder bass volumes (higher sound pressure level or “SPL”) with smaller speakers without additional electronic amplification. Due to the low frequencies and long wavelengths of bass sounds, designers must pay special attention to the overall physical design of the loudspeaker housing, in addition to the selection of the loudspeaker itself. While one may generate treble easily by connecting a high frequency loudspeaker to a power amplifier, generating bass sounds is not so simple. One must consider not only the loudspeaker itself, but also the size, shape and construction of a low-frequency cabinet when designing for a proper bass response. To complicate matters further, the designer must take care to ensure volume levels are consistent across the audio spectrum (i.e, a “flat” response) so that resonances at higher frequencies do not cause “tinny” sounds.
If one simply connects a loudspeaker to an amplifier and tries to generate bass sounds by “pumping up the volume”, several problems occur. First, turning the power of the amplifier up causes the loudspeaker cone to move further in and out, thereby straining the suspension material that ties the loudspeaker frame to the loudspeaker cone. Loudspeakers reproduce sound most accurately when the loudspeaker cone moves little relative to the frame. When the displacement of the cone becomes great relative to the frame, distortions appear in the reproduced sound.
Second, if the loudspeaker is just sitting in free space with no housing about it, the sound waves generated by the back of the cone may cancel those being produced by the front of the cone, thereby reducing the overall output. For example, many prior art low frequency speaker systems comprise a simple low frequency loudspeaker, or “woofer”, with no enclosure, as in television and radio sets and some public address systems. The problem with these systems is that there is no way to prevent the radiation from the back of the speaker from canceling the radiation from the front. Thus, these systems require very large cone displacements at low frequencies.
One prior art approach for reducing back radiation is to place the loudspeaker driver in a closed box to form what is often called an “acoustic suspension system.” In an acoustic suspension system, air trapped inside the closed box provides a reactance against which the loudspeaker cone works. The reactance limits the cone displacement and also prevents the radiation from the back of the loudspeaker from canceling that from the front. Any air leaks in the box will compromise the bass response, in that they both reduce the reactance and allow “cancellation sound” to escape from the box. A large volume within the closed box not only increases the SPL if the bass response, but additionally reduces resonances that may cause tinny sounds. As is known in the art, for any given design, a larger volume results in a lower Helmholtz resonant frequency, improving both bass SPL and overall frequency response provided the box is sealed in an airtight fashion.
The problem with prior art closed box solutions is that while they lend themselves well to large loudspeaker cabinets made from materials like wood, they are difficult to apply to small loudspeaker devices, like speakerphone accessories for cellular phones. Speakerphone accessories are generally small, and are made from materials like plastic that do not form airtight seals very well. Additionally, these accessories often include other circuitry, like charging circuitry for example, that must be coupled to the host electronic device by wires. Running wires through the walls of the accessory causes leaks, thereby compromising acoustic reproduction, especially the bass response.
There is thus a need for a speakerphone accessory with an improved low frequency response.